Sudden Interlacing Problem When Changing Sequence Settings

Hi, sorry for being a n00b (in more ways than one), but I'm suddenly experiecing a strange problem and was hoping some good people here might be able to help me out.

I've actually been using Adobe Premiere for a few years, and I haven't experienced this problem until very recently: if I change the sequence settings for a project I'm working on (namely, adjusting the framerate), the final exported file results in interlaced video. As I said, this has only started recently happening, and here's two examples I can show you where you can see the difference for yourself:

Here's a comedy short I did earlier in the summer, where a portion of the short (specifically, from 0:12 to 5:35) had the framerate adjusted from 29.97 fps to 24 fps:

Now here's a newer video that I finished a few weeks ago and just uploaded this week, and likewise, the framerate was changed from 29.97 to 24:

Before anybody starts to ask questions, I film with an HD camera (a Sony AVCHD Handycam, to be precise), but I've never exactly been a fan of 16:9, which is why I always frame and crop my shots for 4:3, while still maintaining HD quality (it's just 1440x1080, rather than 1920x1080). I'm also a big fan of older shows (and movies) that were shot on 35mm film as opposed to today's digital recording, which is why I always try to give my more of a film look, by adjusting the brightness/contrast, color saturation, grain, and of course framerate . . . but, as you can see in the second video, apparently adjusting the framerate now results in interlaced video footage, which I never experienced up till this point. The video preview in Premiere itself shows no interlacing, and when watching the final product in a media player like Windows Media Player or VLC, the interlacing isn't quite so noticeable, but as you can see when uploaded to YouTube, the interlacing is [i]very[/i] noticeable.

At first, I thought maybe this had something to do with updating to CC 2015 3.3, because I had noticed there were a number of minor differences with CC 2015 3.3 (like certain effects being retired, such as the Fast Color Corrector, which I always used), so I went through the trouble to uninstall CC 2015 3.3 and reinstall the first CC 2015 release . . . but then, I started experimenting and testing, and I found that the problem is still persisting: footage where the framerate is adjusted from 29.97 fps to 24 fps are still interlaced when exported.

The only thing I can think of that might be affecting this was I had to redo my prefered export settings when I updated to CC 2015 3.3 then reverted back to CC 2015, but I'm fairly certain I changed them back to what I had them as when I was originally still using CC 2015 at the time the above-mentioned first video was completed (H.264, with matched sequence settings), but I'm not entirely sure. I also haven't changed any of my camera's settings either - and, on a hunch, I when conducting my tests, I sampled footage shot in both 60i (which is what I've been using all along) and 60p to see if that made a difference, and it didn't - both formats exported with interlacing when the framerate was adjusted.

Does anybody have any idea as to why this is suddenly happening? If more information is needed, I'll try to provide it.